Diesel engines used to drive transport refrigeration equipment produce low frequency tones at their firing frequencies and their harmonics. The refrigeration units can be required to hold the load temperature within 0.1 F.xc2x0 of the set point which may be 40xc2x0 F. for flowers or produce and xe2x88x9220xc2x0 F. for ice cream. A number of these units can be parked and running at cold storage warehouses, interstate highway rest stops, etc. Because these units can be running at various loadings and because the engine speeds of the units are operator adjustable by a couple of percent, the noise outputs will be at different frequencies, but may be relatively coherent such that the various different frequency noise sources cyclically go into and out of phase. As the noise sources go from reinforcing to opposing the other noise sources, there is a perceived varying of the sound level. These tones can be sources of annoyance in the community adjacent areas where a number of units are running.
U.S. Pat. No. 6,009,705 discloses a noise attenuator employing a plurality of quarter wave resonator tubes and Helmholtz resonators. Each will be tuned to a separate narrow frequency range. The effectiveness of the various resonators will drop off as the frequencies of the noise sources vary from the design frequencies as the engine speed/load changes.
A transport refrigeration unit is, typically, driven by a diesel engine. As is conventional for internal combustion engines, ambient air is drawn through a filter into the cylinders of the engine. Rather than connecting one or more attenuators effective over single narrow ranges, the present invention employs an adjustable resonator in an elbowed side branch. Within the resonator is a butterfly or flapper valve that is either fully open or closed. At low engine speed, the valve is open and the sound energy from the engine enters the resonator and is reflected back on itself, 180xc2x0 out of phase. The distance between the closed end of the side branch and the inlet pipe is xcex1/4, where xcex1 is the wavelength of the tone of interest in the inlet pipe during low speed operation of the engine. At high speed, the valve is closed and the resonator is tuned to the higher engine speed, with the distance between the inlet pipe and the closed valve being xcex2/4, where xcex2 is the wavelength of the tone of interest in the inlet pipe during high speed operation of the engine. The valve is connected to the engine speed control such that the valve is positioned in accordance with the engine speed of a two speed engine.
It is an object of this invention to eliminate the need for a resonator for each frequency of interest at both high and low speed operation.
It is another object of this invention to provide a resonator effective in two frequency ranges. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, the tunable resonator is coupled to the engine speed control such that the resonator is set to a different frequency range when the engine speed is changed. The frequency range is changed to a higher frequency by closing a valve which effectively reduces the length of the resonator.